Forming attached features on a semiconductor substrate

ABSTRACT

A method for creating attached features while controlling the depth profile between the features is presented. First the features are formed with a separating barrier between the features. The separating barrier is then etched in a second step with an orientation dependent etchant to attach the two feature. This method can be used to create attached features of relative similar sizes or attached features of disparate sizes.

FIELD OF THE INVENTION

This invention relates generally to the fabrication of semiconductordevices and more particularly to the fabrication of a smaller, shallowfeature adjacent to a deeper feature.

BACKGROUND OF THE INVENTION

Liquid phase etching of semiconductor substrates provides one of thecore technologies of the microelectronics industry. As the demands forsmaller devices and more densely packed designs have increased, newmethods of liquid phase etching have been developed to meet thetechnological challenge of manufacturing these smaller devices.

One feature of certain liquid phase etching processes is that theyproduce different etch rates along different crytallographic directionsin the semiconductor substrate. For example, in an etch process withethylene diamine pyrocatechol (EDP), an etching process which yields anetch rate of 60-70 microns per hour on an Si(100) surface may only etchan Si(111) surface at 1-3 microns per hour. Other orientation dependentetchants, such as KOH, also exhibit a disparity in etch rates forsilicon substrates. The discussion below will focus on the use oforientation dependent etchants on silicon substrates. Those skilled inthe art, however, will understand that this background discussion can bebroadly applied to substrates of other materials which exhibitorientation dependent etching.

Due to the anisotropy of etch rate with respect to crystal direction,orientation dependent etchants (ODEs) are well-suited for creatingpyramidal etch features on (100) surfaces. This is particularly truewhen an etch feature is created by exposing a rectangular portion of a(100) surface to an orientation dependent etchant. If the sides of therectangle are properly aligned with the <110> directions, theorientation dependent etch will produce an etch pit with (111)sidewalls. The angle between the (111) surfaces and the (100) surface is54.7 degrees. Based on this relationship, if the etch is allowed toproceed for a sufficient length of time, an etch feature will beproduced which is defined by the intersection of (111) surfaces. If sucha feature is etched to completion, the depth of the feature will be onehalf of the product of the smallest side of the original exposedrectangle and the tangent of 54.7. This calculated depth excludes anyadditional depth from overetching of the feature. Such overetchingusually results in only a modest increase in depth, however, due to thegreatly reduced etch rate for ODEs on (111) surfaces. This allows for agreat deal of process flexibility, as an etch process may be continuedfor a significant length of time without appreciable overetching of afeature.

Due to the self-limiting nature of this type of etch process, featuresof disparate sizes may often be etched in a single step. As an example,it may be desirable to etch a small feature and a large feature duringthe same step. If both features are exposed to an orientation dependentetchant, less time will be required to form the limiting pyramidal shapeof intersecting (111) surfaces in the small feature. Because of the slowetch rate on (111) surfaces, however, additional overetch will lead toonly a modest amount of additional etching in the small feature. As aresult, allowing the etch to proceed for a longer time in order tocreate a large, deeper feature will not lead to an appreciable change inthe smaller etch feature. Thus, smaller and larger features can becreated during a single etch step.

While this technique for creating smaller and larger features during asingle etch step may be applicable in many situations, problems arisewhen it is desirable to etch a small, shallow feature, such as a notch,which is attached to a larger, deeper feature. FIGS. 1-3 schematicallyshows the difference between features which are near to each other andfeatures which are attached. In FIG. 1, features 20 and 22 are only nearone another. The height of barrier 24 between the features issubstantially the same as the height of the substrate material 28. Onthe other hand, features 30 and 32 in FIG. 2 are attached. Although abarrier 34 is still present in the transition region between features 30and 32, the height of barrier 34 is well below the height of thesubstrate material 38. Features 40 and 42 in FIG. 3 are also attached,although now no barrier is present. Instead, features 40 and 42 arejoined via transition region 44.

For example, it may be desirable to control the shape and the depth ofboth a large feature and an attached small feature with a depth profilelike the one shown in FIG. 3. The large feature and the attached smallfeature cannot be etched in the same step without losing control overthe shape of at least one of these features. Because the features areattached, (111) surfaces will be unable to form in the transition regionbetween the large feature and the attached small feature. Instead, avariety of surfaces will be exposed that etch rapidly in an ODE. Thiswill lead to fast etching in the transition region and at the bottom ofthe small feature, resulting in a deeper small feature than desired andloss of a great deal of material from the transition region and thesidewall of the large feature where it meets the transition region.

One alternative to etching the large feature and attached small featurein a single process step is to etch the large feature during a firstetch step and then to etch the entire attached small feature, includingthe transition region, during a second etch step. Splitting the etchinto these two steps provides some additional control over the process,but difficulties still remain. A variety of crystallographic faces arepresent in the transition region between the attached small feature andthe large feature. If the transition region is exposed to an ODE, theetch rate in the transition region will be faster than the etch rate ona (111) surface. As a result, it is difficult to control the etch depthin both the transition region and the attached small feature at the sametime.

In U.S. Pat. No. 4,601,777, a method for creating a small channel near alarge cavity is disclosed for use in making ink jet printer heads. Themethod involves a two step etching process. In the first step, the largecavity and the small channels are created using an anisotropic etch,such as KOH, but a barrier is left between the cavity and the channel.In the second step, the barrier is removed by either dicing, amechanical means, or by an isotropic etch followed by an anisotropic, ororientation dependent, etch. The isotropic etch used in the invention isa mixture of HF, HNO₈, and C₂H₄O₂. This disclosure also notes theunsuitability of using only an orientation dependent etch to remove thebarrier between the cavity and the channel.

U.S. Pat. No. 4,810,557 discloses a method for creating a tandemV-groove where a portion of the groove is shallow and narrow relative tothe remainder of the groove. In this method, the attached shallow grooveand deep groove are created by applying two etch masks. The pattern inthe first mask determines the location of the grooves. The second maskis then deposited to prevent etching in the shallow groove during theinitial stages of the etch.

U.S. Pat. No. 4,863,560 discloses a method for creating both large,coarse features and small, fine features without having to resort tolithography steps in between etching steps. Several etch masks aredeposited prior to etching, with the mask for the largest, coarsestfeatures at the top of the stack of masks. The masks are patterned asthey are deposited without etching the underlying substrate. After allof the etch masks are formed, the first etch process is carried out.After the first etch process the coarse mask is removed, leaving behinda second etch mask. This process can be repeated to achieve finer andfiner control over the etch features created. U.S. Pat. Nos. 5,131,978and 5,277,755 disclose other methods for using multiple etch masks tocreate attached small and large features.

U.S. Pat. No. 4,957,592 discloses a method for creating small and largefeatures at the same time using an erodable etch mask. The erodable etchmask is formed and patterned on the substrate. A non-erodable etch maskis then formed and patterned above the erodable layer. During the etchstep, the erodable mask is slowly consumed by the etchant. As a result,the areas of the substrate covered only by the erodable mask are notetched initially, but are eventually uncovered. This allows for the etchto start in the areas covered by the erodable mask at a later point intime than the rest of the substrate, leading to an effectively shorteretch time for the areas covered by the erodable mask.

U.S. Pat. No. 5,096,535 discloses a method for creating small featuresattached to large features. The etching mask is patterned to expose thesurface areas of the small and large features while leaving barriersbetween the features. These barriers between the features are consumedduring the same etch step as the features by undercutting the etch mask.

Restricting etching of the barrier to undercutting of the etch mask, asin U.S. Pat. No. 5,096,535, may overcome some of the aforementionedproblems associated with attempting to create both the large and smallfeature in a single step. To the degree that this is true, however, thistechnique introduces a new problem. If an ODE etch process is used toboth create the small feature and remove the wall, the etch will proceedquickly on (100) surfaces and slowly on (111) surfaces. As (111)surfaces will quickly form in the small feature, removal of materialfrom the ‘wall’ will proceed based on the etching rate of (111)surfaces. The slow etch in this direction may result in creation of a‘bump’, or high point, in the transition region between the smallfeature and the large feature.

The slow etch rate of orientation dependent etchants on (111) surfaceshighlights another difficulty in the current state of the art. In someapplications, it may be beneficial to create two attached features whereboth features are relatively deep. In this case, the transition regionbetween the feature could resemble the barrier shown in FIG. 2. Usingthe undercutting technique described above in U.S. Pat. No. 5,096,535, abarrier between two attached features can be consumed during the sameprocess step used to form the two features. In this undercuttingtechnique, however, the barrier is consumed due to etching (111)surfaces. While this will eventually lead to etching of the barrier,removal of more than a few microns from the barrier will take hours. Ifthe height of the barrier must be reduced by more than just 1-3 microns,it would be desirable to have a method where the barrier could be etchedat a much higher rate. This method, however, should also retain the(111) surface character of the barrier walls for any portions of thebarrier which remain after the etch.

Accordingly, there is a need for an improved method of creating a smallfeature which is attached to a larger feature. The method should allowfor creating the small attached feature without significantly degradingthe sidewall depth profile of the small feature or the large feature.The method should avoid the difficulties of forming multiple etch maskson a substrate prior to etching. The method should also be relativelysimple to implement and reproducible for use in production ofmicroelectronic devices.

There is also a need for a method of creating large attached featureswith a barrier in the transition region. The method should be able toquickly and efficiently create the attached features. The method shouldalso produce a barrier having sidewalls with (111) surfaces.

SUMMARY OF THE INVENTION

The present invention provides a method for creating a small featureattached to a large feature while maintaining control over the depthprofile of the transition region between the features. In an alternativeembodiment, the method allows for creation of attached features ofsimilar sizes while maintaining control over the depth profile of thetransition region. In the inventive method, a substrate is providedwhich has one or more protective layers over the substrate. Typicallyone protective layer of SiO₂ is present, but layers of other materialssuch as Si₃N₄ may also be used as protective layers. The substrate istypically composed of silicon, but other substrate materials may be usedif a suitable orientation dependent etchant is available for thematerial.

A first layer of patterned photoresist is formed on the substrate. Thisfirst patterned photoresist layer provides definition for a smallfeature, a large feature, and a separating barrier between the smallfeature and the large feature, as well as pattern definitions for anyother features which it is desirable to transfer to the substrate. In analternative embodiment, the first patterned photoresist layer providesdefinition for two features separated by a barrier, as well as patterndefinitions for any other features which it is desirable to transfer tothe substrate.

After forming the patterned photoresist layer, successive steps transferthe pattern first to the protective layer or layers and then to thesubstrate. The exact nature of the pattern transfer step depends on thematerial which must be etched to accomplish the transfer. For an SiO₂protective layer, the pattern transfer is carried out by exposing thewafer to a buffered oxide etch solution. Pattern transfer into thesubstrate is accomplished by exposing the substrate to an orientationdependent etchant, such as EDP or KOH. The patterned photoresist may beremoved at any time after the first of the transfer steps.

Once the first pattern is transferred into the substrate, a second layerof patterned photoresist is formed on the surface. Now that the surfaceis no longer planar, a spray-on technique for photoresist application ispreferred. Other photoresist application techniques such as lamination,spincoating, or electrophoretic deposition may also be used if asuitable degree of planarity control can be achieved. The pattern in thephotoresist provides for exposure of the protective layer or layerswhich reside on the separating barrier created by the previous patterntransfer steps.

The exposed protective layer or layers above the separating barrier arethen etched away. Once again, the nature of this removal step depends onthe composition of the protective layers. For example, the wafer couldbe exposed to a buffered oxide etch solution to remove SiO₂, or thewafer could be exposed to hot H₃PO₄ for removal of Si₃N₄. After removalof the protective layers, the separating barrier itself is etched withan orientation dependent etchant, leading to formation of the transitionregion and production of the final structure of a small feature attachedto a large feature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a substrate with twofeatures which are near each other.

FIG. 2 is a schematic cross-sectional view of a substrate with twofeatures which are attached.

FIG. 3 is a schematic cross-sectional view of a substrate with twofeatures which are attached.

FIG. 4 is a schematic cross-sectional view of a substrate suitable forprocessing with an embodiment of the invention.

FIG. 5 is a schematic cross-sectional view of La substrate undergoingthe process of an embodiment of the invention.

FIG. 6 shows the substrate of FIG. 5 at a processing step subsequent tothat shown in FIG. 5.

FIG. 7 shows the substrate of FIG. 5 at a processing step subsequent tothat shown in FIG. 6.

FIG. 8 shows the substrate of FIG. 5 at a processing step subsequent tothat shown in FIG. 7.

FIG. 9 shows the substrate of FIG. 5 at a processing step subsequent tothat shown in FIG. 8.

FIG. 10 schematically represents a sample depth profile of a substrateprocessed according to the invention.

FIG. 11 is a schematic cross-sectional view of a substrate undergoingthe process of an alternate embodiment of this invention.

FIG. 12 shows the substrate of FIG. 11 at a processing step subsequentto that shown in FIG. 11.

FIG. 13 shows the substrate of FIG. 11 at a processing step subsequentto that shown in FIG. 12.

FIG. 14 schematically depicts a laser module on a substrate created inpart with the present invention.

FIG. 15 schematically depicts a portion of the substrate from FIG. 14 ingreater detail.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that structural, logical, chemical, and electrical changesmay be made without departing from the spirit and scope of the presentinvention.

The terms “wafer” and “substrate” are to be understood as including anysemiconductor-based structure which may be beneficially treated by theprocess of this invention. “Wafer” or “substrate” may includesilicon-on-insulator (SOI) or silicon-on-sapphire (SOS) technology,doped and undoped semiconductors, epitaxial layers of silicon supportedby a base semiconductor foundation, and other semiconductor structures.Furthermore, when reference is made to a “wafer” or “substrate” in thefollowing description, previous process steps may have been utilized toform regions or junctions in the base semiconductor structure orfoundation. In addition, the semiconductor need not be silicon-based,but could be based on silicon-germanium, germanium, or gallium arsenide.The substrate could also be based on indium phosphide or other materialsfor which a suitable orientation dependent etchant is available.

In the following detailed description, reference will be made to anumber of materials with variable stoichiometry. Materials such asoxides or nitrides of silicon ideally have chemical compositions of SiO₂and Si₃N₄, respectively. In actual use, however, oxides and nitrides ofsilicon may depart from this ideal stoichiometry. When reference is madeto a material of potentially variable composition, such reference shouldbe understood to encompass the ideal stoichiometry as well as variationsfrom the ideal structure. Thus, references to SiO₂, silicon oxide, or anoxide of silicon are interchangable, as are references to SiN₄, siliconnitride, or a nitride of silicon.

The present invention provides a method for creating a small featureattached to a larger, deeper feature while maintaining control over theprofile of the transition region between the small and large feature.The method is generally applicable to features with sufficient sizedisparity. The inventive method will allow for creation of a smallfeature attached to a large feature which may have about 10 times thesurface area of the small feature, about 25 times the surface area ofthe small feature, or even greater surface area disparities. Further,the maximum depth of the transition region is less than about 25% of thedepth of the large feature and the minimum depth of the transitionregion is greater than the maximum depth of the small feature. Note thatwhen the small feature and large feature are created with orientationdependent etchants, the initial size of the opening used to create thefeature roughly correlates with the depth of the feature if the featureis etched to termination. A feature is etched to termination when the(111) surfaces of the feature sidewalls intersect at the bottom of thefeature. If a feature is not etched to termination, the bottom of thefeature will consist of a (100) surface, or other surface, whichintersects with the (111) surfaces of the sidewalls.

Referring now to the drawings, where like elements are designated withlike numerals, FIG. 4 schematically depicts a semiconductor substrate 50at an intermediate processing stage. Substrate 50 may already havevarious devices or features on top surface 52, such as bond pad 54 ormetal lines (not shown). In this example, substrate 50 is primarilycomposed of silicon while protective layer 56 is composed of SiO₂. Otherfeatures may already be embedded in the substrate, such as a via forbonding lower levels of metallization to a surface metal layer ordevice, or a cavity for holding an object, such as a mirror or anothercomponent of a laser assembly. In a preferred embodiment, however, theinitial substrate surface is substantially planar. This allows for useof conventional spin-on techniques during application of the firstphotoresist layer.

In areas where a feature is not already present, such as the eventuallocation of the attached small and large features, top surface 52 is apart of protective layer 56, which is typically composed of a materialsuch as SiO₂. Other materials, such as Si₃N₄, may also be suitable aswill be apparent from the nature and scope of this invention. Protectivelayer 56 resides above substrate 50. Although FIG. 4 depicts a singlelayer 56 of protective material above substrate 50, multiple top layersof protective material could be present above the substrate. Substrate50 could also be composed of other materials, such as Ge or GaAs, solong as chemistries are available for orientation dependent etching ofthe alternative materials. If alternative materials are present, thespecific method steps used to create the attached small and largefeatures may differ due to the need for alternative etching chemistries,but the spirit of the invention will remain the same.

FIG. 5 schematically depicts the portion of substrate 50 in FIG. 4 wherethe large feature and the attached small feature will be created. Thesubstrate is now at a later stage of processing. A layer of photoresist60 has been formed on top surface 52. This photoresist has beenpatterned to leave gap 62 in the photoresist for definition of the smallfeature and gap 64 for the definition of the large feature. Gaps 62 and64 are separated by a photoresist wall 66. The SiO₂ or other exposedmaterial of top surface 52 can now be etched through gaps 62 and 64. Ablanket photoresist layer (not shown) is also formed on the bottomsurface of the substrate to protect the bottom surface. The bottomsurface photoresist layer is not patterned as no processing is requiredon the bottom surface of the wafer.

Note that the bottom surface photoresist layer is only necessary if thebottom surface is composed of a material which will be susceptible toattack by the etchants used to treat the top surface of the wafer. Forexample, if the bottom surface of the wafer is composed of SiO₂, thephotoresist layer is only necessary if the substrate will be treatedwith an etchant which attacks SiO₂.

Next, the pattern is transferred from photoresist layer 60 to theunderlying substrate. The exact form of this step will depend on thenature of the top surface of the substrate. The substrate will becovered with one or more protective top layers 56, such as a layer ofSiO₂. As a result, two or more process steps will be necessary forcomplete transfer of the pattern into the substrate.

In the first pattern transfer steps, exposed areas of protective layer56 are removed with a suitable etchant. For example, in the case of anSiO₂ top layer, a BOE solution may be used to remove the oxide layer. Inthe case of an Si₃N₄ protective layer, a solution of hot phosphoric acidmay be used to remove the nitride layer. In a preferred embodiment, asubstrate with an exposed SiO₂ protective top layer is treated with aBOE solution at approximately 20° C. In other embodiments, the substrateis treated with a BOE solution at a temperature between approximately15° C. and approximately 25° C. The process of removing exposedprotective layers is repeated until all of these layers have beenremoved in exposed areas. Once all of the protective layers are removed,a final pattern transfer step using an orientation dependent etchant iscarried out to transfer the pattern to substrate 50.

Once the initial pattern transfer step has occurred between thephotoresist layer 60 and the substrate 50, any remaining photoresist maybe removed from the substrate, including photoresist present on thebottom surface of the layer. For example, if a wafer comprises an SiO₂top layer over a silicon substrate, the photoresist may be removed oncethe pattern has been transferred to the SiO₂ top layer. The photoresistmay be removed by any suitable technique which is compatible with theexposed surfaces on the substrate. Such techniques could includeexposure of the substrate to organic stripping solutions, ozonatedwater, or oxygen ashing.

In general, the final pattern transfer step will involve exposing thesubstrate to an orientation dependent etchant. If the substrate 50 iscomposed of silicon, suitable orientation dependent etchants includeethylene diamine pyrocatechol (EDP) and strong bases which donate OH—,such as KOH. Additional care should be taken when KOH is used as theorientation dependent etchant, as KOH will also etch SiO₂, a commonprotective layer material, at an appreciable rate. In alternativeembodiments, solutions involving hydrazine may also be used. Because EDPand KOH are orientation dependent etchants, the etch rate of silicon inthese etchants depends on the particular crystalline facet which isexposed to the etchant. For example, typical etch rates for silicon inEDP at 107° C. are 60-70 microns per hour on a (100) surface, but only1-3 microns per hour on a (111) surface. In other embodiments, asubstrate might be exposed to EDP at temperatures between approximately100° C. and approximately 115° C. Temperatures above 115° C. may beimpractical due to the fact that EDP boils at this temperature. On theother hand, as the temperature of the EDP solution is reduced, thesolubility of etch products in the EDP solution drops to levels whichcan lead to processing problems. As noted above, orientation dependentetchants for silicon are particularly suitable for creating pyramidaletch features in a silicon substrate.

The substrate is exposed to the orientation dependent etchant for a timesufficient to create small feature 70 and large feature 72, as shown inFIG. 6. Due to the self-limiting nature of the orientation dependentetch, overexposing the small feature leads to only a minimal amount ofadditional etching. Generally the etch will be performed for a timesufficient to completely form (111) surfaces within the small feature.For the large feature, the etch may proceed for a sufficient time toalso completely form (111) surfaces in the large feature, or the etchmay be stopped prior to this point. If the etch process is terminatedbefore fill formation of (111) surfaces in the large feature, the largefeature will be composed of (111) surfaces which terminate in a flat(100) bottom surface. In either case, other features may be created onthe substrate during this etch process.

FIG. 6 also depicts separating barrier 74 and barrier cap 76, whichremain between small feature 70 and large feature 72 after completion ofthe pattern transfer step. In this example substrate, separating barrier74 is composed of silicon while barrier cap 76 is composed of SiO₂.Although barrier cap 76 on top of separating barrier 74 protects thebarrier from direct attack by the etchant, separating barrier 74 isstill etched slowly due to undercutting of protective layer 56 andbarrier cap 76 during formation of the small and large feature. This isrepresented in FIG. 7 by the small overhang of protective layer 56 oversmall feature 70 and large feature 72. In a preferred embodiment, thefinal width of separating barrier 74 after the pattern transfer stepwill be between about 1 micron and about 14 microns. Thus, the initialspacing in FIG. 5 between gaps 62 and 64 in the patterned photoresistlayer 60 used to form separating barrier 74 must be adjusted to accountfor the loss of material which will occur due to undercutting of barriercap 76 during the pattern transfer.

Note that FIG. 6 represents the continuation of processing on a samplesubstrate. In other embodiments, alternative materials could be selectedfor the protective layer, such as Si₃N₄. In these embodiments, barriercap 76 would be composed of the alternative protective layer material.In still yet other embodiments, the substrate material could be amaterial other than silicon.

Once the small feature 70 and large feature 72 are formed, a secondlayer of patterned photoresist is formed on the substrate. Due to thefact that a relatively large feature is now present on the surface ofthe substrate, traditional spin-on techniques for application ofphotoresist may not be effective. An alternative method for applyingphotoresist at this point is to spray on the photoresist. In a preferredembodiment, photoresist is applied to the substrate by first preparing asolution of Shipley 1818 photoresist which is thinned with a thinner,such as Shipley Type P thinner. This thinner is primarily composed ofpropylene glycol monomethyl ether acetate. In a preferred embodiment,the thinned photoresist solution is composed of approximately 70%Shipley Type P thinner, approximately 30% Shipley 1818 photoresist, andapproximately 0.2% FC 430. This last component is a flow control agentavailable from the 3M corporation. This thinned photoresist is thensprayed on to the surface of the wafer. Other methods for applyingphotoresist in this circumstance will be apparent to those skilled inthe art. After soft-baking the top photoresist layer, a photoresistlayer is also applied to the bottom surface of the wafer and thensoft-baked.

After the soft-bake, this second photoresist layer is patterned. FIG. 7shows second photoresist layer 80 after patterning on the samplesubstrate. In FIG. 7, the photoresist on top of barrier cap 76 has beendeveloped away. As a result, barrier cap 76 is now susceptible tochemical treatment while the remainder of the surface is protected withphotoresist.

After patterning second photoresist layer 80, the separating barrier 74is removed via etching. As before, the exact nature of the barrierremoval etch process will depend on the type of substrate material andthe number of protective layers present on the surface. FIG. 8 depictsthe start of the wall removal etching process for the sample substrate.After patterning second photoresist layer 80, the substrate is treatedwith BOE. The BOE removes the exposed SiO₂ barrier cap 76. Once again,in a preferred embodiment the substrate is treated with the BOE at 20 C.In alternative embodiments, the substrate could be treated with BOE atother temperatures, especially between approximately 15 C. and 25 C.This exposes separating barrier 74 for additional processing. SiO₂ inother areas of the surface is largely unaffected due to the presence ofsecond photoresist layer 80 as well as the blanket layer of photoresistformed on the bottom surface of the wafer. After removal of barrier cap76, second photoresist layer 80 and the bottom surface photoresist layerare stripped from the surface by a suitable method. The structure shownin FIG. 8 is now ready for removal of separating barrier 74 in order toallow attachment of small feature 70 and large feature 72.

The substrate is then exposed to an orientation dependant etchant, suchas EDP. Due to the nature of the structure of separating barrier 74, theetch rate of separating barrier 74 is greater than the typical etch ratefor a (100) surface. At 107° C., the etch rate of separating barrier 74is typically between about 250 microns per hour and about 300 micronsper hour. FIG. 9 shows the resulting substrate where separating barrier74 has been removed. Removal of separating barrier 74 has resulted information of transition region 82 between small feature 70 and largefeature 72. In FIG. 9, transition region 82 is defined as the regionbeginning at the bottom of the small feature 70 and ending at the pointwhere the sidewall of large feature 72 is once again a (111) surface.

In other embodiments, the etch process for removal of the separatingbarrier will vary to account for different substrates. For example,instead of a barrier cap 76 composed of SiO₂, the separating barrier maybe protected by a barrier cap composed of Si₃N₄, requiting analternative etch chemistry such as hot phosphoric acid.

FIG. 10 depicts a typical profile of the depth of the surface tracedalong a line starting in the small feature, through the transitionregion, and into the bottom of the large feature. Plateau 90 shows thefinal depth of the small feature. First transition point 92 and secondtransition point 94 identify positions in the transition region betweenthe small feature and the large feature. Sidewall region 96 is theexpected (111) surface formed in the large feature by the orientationdependent etch. Finally, bottom plateau 98 represents the bottom oflarge feature. As noted above, the transition region begins at the edgeof plateau 90 and continues until the second transition point 94, wherethe sidewall becomes a (111) surface. Note that the bottom of the smallfeature is represented as a plateau 90 for ease of visual inspection.FIG. 10 only indicates the maximum depth of the small feature withoutnecessarily indicating the nature or shape of that feature.

Note that the method of the present invention allows for reproduciblecontrol over both the depth and width profiles on the surface where thebottom of the small feature intersects with the large feature. The smallfeature can be overetched safely due to the limiting nature oforientation dependent etching. Efforts to etch the small feature at thesame time as the separating barrier, however, have led to problems withreproducibility. If the etch is allowed to proceed for too long, thetransition region between the small and large feature will continue toetch. In the limiting case, this can result in the small feature havingthe same depth as the large feature. On the other hand, if the etch isterminated too quickly, a ‘bump’ may be left in the transition region.The presence of a bump could cause the bottom of the small feature toactually be deeper than some portions of the transition region. Inapplications such as the laser module described below, the presence of abump in the transition region can lead to partial blocking or refractingof laser light which is attempting to pass through the attached smallfeature to a device located in the large feature.

After completion of the inventive method, several additional steps maybe taken. As noted above, the long etch required for creation of thelarge feature can result in overhang of the protective layers over thesmall feature and large feature. These overhangs can be removed by anysuitable method. Another possible follow-up step would be to rinse thewafer after formation of the small feature and the attached largefeature. The rinse could include exposure of the substrate to deionizedwater. Additional intermediate steps might also be included during theinventive process. For example, a rinse might be included after creationof the small and large etch features but prior to formation of thesecond photoresist layer. Other additional intermediate or follow-onsteps, such as alternative rinses, will be apparent to those skilled inthe art.

While the above embodiments describe use of the present inventive methodfor creating attached features of disparate sizes, this inventive methodmay also be used generally for creating attached features of any size ona surface. In particular, the present method can be used to greatadvantage in creating attached features of similar sizes when it isdesirable for both of the attached features to be relatively deep. Thenumber and types of process steps used to create the attached featuresof similar size will be basically the same as the process steps used tocreate a small feature attached to a large feature. The depth profile inthe transition region between the features, however, will be different.

The method for creating attached features of similar sizes generallyresembles the method for creating a small feature attached to a largefeature. First, a layer of patterned photoresist 104 is formed on asubstrate, as shown in FIG. 11. As before, substrate 100 comprises oneor more protective layers 102 over an orientation dependent etchingmaterial. The primary difference between the substrate shown in FIG. 5and the substrate shown in FIG. 11 is that gaps 106 and 108 in FIG. 11,where the features will be formed, are about the same size. This is incontrast to small gap 62 and large gap 64 in FIG. 5, which are designedfor formation of a small feature and large feature, respectively.

After forming patterned photoresist layer 104, the pattern istransferred, first to protective layer 102, and then to substrate 100with an orientation dependent etchant as before. This results increation of two features 110 of approximately the same size separated bya barrier, as shown in FIG. 12. At this point, a rinse may be insertedas described above. Next, the second layer of patterned photoresist 112is formed on the surface. Once again, the pattern is designed to allowetching of the barrier 114 between the features.

Once the photoresist is patterned, barrier cap 116 above the barrier isremoved. Barrier 114 is then etched with an orientation dependentetchant. In this case, the goal is to remove only a portion of thebarrier while retaining as much of the (111) surface character of thebarrier sidewalls as possible. The final structure after etching thebarrier is depicted in FIG. 13.

When creating features of approximately equivalent size, the depthprofile between the attached features will differ from that for featuresof disparate sizes. Since both features will have similar initialsurface areas, both features will also achieve the limiting shape ofintesecting (111) surface sidewalls at roughly the same time. As aresult, the bottom surfaces of the two attached features will haveapproximately the same depth.

When etching features of similar size, the desired structure afteretching the barrier is to reduce the height of the barrier to a desiredlevel while retaining the (111) surface character of the featuresidewalls for the portion of the barrier which remains after the etch.This latter goal is achieved by using an orientation dependent etchantto remove a portion of the barrier. Because surfaces other than (111)surfaces are exposed at the top of the barrier, however, the orientationdependent etchant can remove the barrier at a much faster rate thanwould be possible by simply undercutting an etch mask. Thus, the barrieris removed quickly while retaining the (111) surface character of theremaining barrier sidewalls in the features.

For a specific example of the inventive method, consider a laser andball lens module. FIG. 14 schematically shows the final substrate withlaser 120 bonded to bonding pad 122. During operation of the laser,laser light passes from laser 120 through notch 124, a small feature, toball lens 126 which resides in ball lens cavity 128, a large feature.

As depicted in FIG. 15, a closer schematic view of the large feature andattached notch, ball lens 126 is held in place in ball lens cavity 128in part due to bonding to sidewall 130 at attach point 132. The sidewall130 shown in FIG. 15 is the wall of ball lens cavity 128 where the notch124 is attached. The attach point 132 is on a (111) surface formed dueto the orientation dependent etching of the ball lens cavity 128. Thisattach point is critical for providing proper bonding of the ball lens126 to the substrate. If the (111) surface of sidewall 130 is notpreserved near the location of attach point 132, ball lens 126 will notbe able to contact the sidewall 130, leading to poor bonding to thesurface. Thus, control of the profile between notch 124 and the balllens cavity 128 is necessary to insure proper bonding of ball lens 126.

The required ball lens cavity 128 and attached notch 124 can be readilycreated by employing the present invention. In this example, a siliconsubstrate has already been processed to create devices and metal levelson the substrate. Metal bond pads and other metallic features areexposed on some portions of the top surface. The remainder of the topsurface is covered with an SiO₂ layer with a thickness of approximately5000 Angstroms. The bottom surface of the substrate is also composed ofa layer of SiO₂.

In this example, the desired ball lens cavity is a pyramidal cavity witha rectangular surface opening of approximately 962 microns byapproximately 2,340 microns and a desired etch depth of approximately450 microns. The attached notch has a desired surface opening ofapproximately 100 microns by approximately 100 microns. The size of thesurface opening for the attached notch in the first patternedphotoresist layer, however will be reduced by the amount needed tocreate the separating wall. Thus, the desired width for the smallfeature prior to removal of the separating wall will be betweenapproximately 99 microns and approximately 86 microns. In the transitionregion, the final depth of the second transition point should be nogreater than 170 microns.

A blanket layer of photoresist is applied to the surface. Thephotoresist is then patterned in the desired locations of the ball lenscavity and the notch. The photoresist is also patterned to define anyother surface features which may be beneficially created during asubsequent silicon etch. A blanket layer of photoresist is also appliedto the bottom surface of the wafer to protect the oxide layer on thebottom surface.

The substrate is then treated with BOE to transfer the pattern from thephotoresist layer into the SiO₂ layer. The substrate is exposed to theBOE at room temperature, resulting in an SiO₂ etch rate of approximately700 Angstroms per minute on any area of the substrate where SiO₂ ispresent on the top surface and is not protected by photoresist. Theprocess is continued for a period of time to remove all of the SiO₂ inthe exposed areas. The substrate may be treated with BOE by immersion ina bath of BOE. Other methods of treating the substrate with BOE, such asspraying, will be apparent to those skilled in the art. Other methods ofremoving the SiO₂ layer from the patterned areas may also be suitable.

After exposing the wafer to BOE, the remaining photoresist on the wafermay be removed by any suitable method. In a preferred embodiment, thewafer is treated with a photoresist stripper such as ACT-150I fromAshland Chemical Co. In another embodiment, the photoresist is consumedby ashing in an oxygen plasma. In yet another embodiment, thephotoresist may be removed by treating the substrate with acetonefollowed by a methanol rinse. Other methods of removing photoresist fromthe wafer will be apparent to those skilled in the art.

After removing the photoresist, the substrate is treated with an EDP atsolution at approximately 107 C. to transfer the features into thesilicon. One possible choice of EDP solution is PSE-300F from theTransene Co. Due to the large size of the ball lens cavity, this etchstep is performed for approximately 7 hours. This is sufficient tocompletely form (111) surfaces in the notch, but a (100) surface isstill present in the bottom of the ball lens cavity. At the end of theEDP etch, the notch and ball lens cavity are still separated by asilicon wall with an SiO₂ wall cap.

After creating the notch and ball lens cavity, a new layer ofphotoresist is deposited on both the top and bottom surfaces of thewafer. The photoresist on the bottom surface may be deposited bytraditional spin-on techniques or other suitable methods. Due to thedeep etch pit on the front side of the wafer, however, it is preferableto spray the photoresist on to the front side of the substrate. Asuitable photoresist for spray-on application is a Shipley 1818photoresist thinned with a Shipley thinner. In a preferred embodiment,the spray-on photoresist is a solution composed by weight ofapproximately 70% Shipley Type P thinner, approximately 30% Shipley 1818photoresist, and approximately 0.2% FC 430 flow control agent from 3MCo. This photoresist layer is then soft-baked.

After forming the photoresist layer on the surface, the photoresist ispatterned to expose the SiO₂ cap of the silicon wall between the notchand the ball lens cavity. Once again, the wafer is exposed to BOE atroom temperature by a suitable method, such as immersion, to remove theexposed SiO₂ wall cap. After removing the SiO₂ wall cap, the remainingphotoresist on both sides of the wafer is removed by a suitable method,such as stripping by exposure of the substrate to ACT-150I from AshlandChemical Co.

After removing the photoresist, the substrate is once again treated withan EDP solution at 107° C. Only the silicon underneath the SiO₂ wall capis etched appreciably by the EDP solution. The oxide layer acts as aprotective mask for any non-exposed silicon and the EDP solution doesnot affect metallic features such as bond pads. Silicon is also exposedto the EDP solution within the notch and other features, but almost allof these features already have the limiting (111) planes exposed, so theetch rate is minimal. The exception to this is the ball lens cavity,where a (100) face is still exposed in the bottom of the cavity. The EDPsolution continues to etch this feature as usual.

This second EDP etch is performed for 15 minutes. At the end of thisetch, the silicon wall between the notch and the ball lens cavity hasbeen removed. The majority of the sidewall of the ball lens cavity,however, has been retained. This will eventually allow for properattachment of the ball lens to the substrate when the ball lens isbonded to the surface.

As can be seen from the embodiments described herein, the presentinvention encompasses a method for creating attached features in asubstrate while maintaining a desired depth profile in the transitionregion between the features. In one embodiment, the method allows forcreation of a small feature attached to a larger, deeper feature. Inanother embodiment, the method allows for creation of attached featuresof similar size with a reduced height barrier between them. The featuresare created by forming a first patterned photoresist layer andtransferring this pattern into the protective top layers andintermediate layers. A separating barrier remains between the featuresafter the first etch step. A second patterned layer of photoresist isthen formed to selectively expose only the area above the separatingbarrier. An initial etch is performed to remove the barrier cap ofprotective layer material from above the separating barrier, followed byremoval of the remaining photoresist. The separating barrier is thenremoved by etching with a suitable orientation dependent etchant. In oneembodiment, the method reproducibly yields a structure where a smallfeature is attached to a larger feature with a controlled depth profilein the transition region between the small and large feature. In anotherembodiment, the method reproducibly yields a structure of two attachedfeatures with a reduced height barrier with a controlled depth profilebetween them.

The above description and drawings are only illustrative of preferredembodiments which achieve the objects, features and advantages of thepresent invention. It is not intended that the present invention belimited to the illustrated embodiments. Any modification of the presentinvention which comes within the spirit and scope of the followingclaims should be considered part of the present invention.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A microelectronic structure in a semiconductorsubstrate comprising: a small feature formed into said substrate to apredetermined depth and having sidewalls which are (111) surfaces; alarge feature formed into said substrate to a predetermined depth andhaving sidewalls which are (111) surfaces, wherein said large feature isat least about 10 times as deep as said small feature; and a transitionregion between said small and large features, wherein said transitionregion has a maximum depth in said substrate which is less than abouthalf the depth of said large feature.
 2. The structure of claim 1,wherein said large feature is at least about 25 times as deep as saidsmall feature.
 3. The structure of claim 1, wherein the bottom of saidlarge feature is defined by the intersection of sidewalls which are(111) surfaces.
 4. The structure of claim 1, wherein the bottom of saidlarge feature is defined by a (100) surface bounded by sidewalls whichare (111) surfaces.
 5. The structure of claim 1, wherein the maximumdepth of the transition region is less than about 25% of the depth ofthe large feature.
 6. The structure of claim 1, wherein the minimumdepth of the transition region is greater than the maximum depth of thesmall feature.
 7. A microelectronic structure for use in a laser modulecomprising: a large feature for holding a ball lens, wherein thesidewalls of the feature are (111) surfaces; a small feature to providea pathway for laser light, wherein the sidewalls of the feature are(111) surfaces; a metal bonding pad in close proximity to the smallfeature as a mounting point for a laser; and a transition region wherethe small feature and large feature are attached, wherein saidtransition region has a maximum depth which is less than half of thedepth of the large feature.
 8. The structure of claim 7, wherein saidlarge feature is about ten times as deep as said small feature.
 9. Thestructure of claim 7, wherein said large feature is about 25 times asdeep as said small feature.
 10. The structure of claim 7, wherein thebottom of said large feature is defined by the intersection of sidewallswhich are (111) surfaces.
 11. The structure of claim 7, wherein thebottom of said large feature is defined by a (100) surface bounded bysidewalls which are (111) surfaces.
 12. The structure of claim 7,wherein the maximum depth of the transition region is less thanapproximately 25% of the depth of the large feature.
 13. The structureof claim 7, wherein the minimum depth of the transition region isgreater than the maximum depth of the small feature.